Flowmeters are presently dominated by designs using an orifice as the flow impedance device. Obstruction flowmeters of this type typically use a baffle plate having a circular opening installed transversely in the conduit. The fluid is forced to flow through the orifice, creating a pressure drop across the plate. The differential pressure is then measured across the orifice, which, along with stream pressure and temperature, are used to compute the flow rate with formulas as known in the art. Alternatively, the flow rate may be computed more accurately by also measuring the temperature differential across the orifice. One such flow rate measuring device is described in U.S. Pat. No. 4,562,744, issued to Hall et al., incorporated herein by reference.
Although the design and construction of a single orifice is simple, its accuracy is highly sensitive to upstream and downstream flow conditions. Consequently, devices commonly called flow conditioners or straighteners are required to eliminate any swirling or other flow field disturbances in the fluid flow upstream of the orifice. In addition, long straight meter runs are required upstream of the flowmeter to reduce unfavorable flow conditions which may affect the accuracy of the flowmeter as well as straight runs downstream.
Another drawback in introducing the normal orifice in the path of fluid flow is that the progression of the fluid is significantly disrupted. While it is desirable to minimize the area of the orifice, such a minimization increases the pressure drop across the orifice. A significant distance downstream from the orifice may be required for maximum recovery of this pressure.
Therefore, it is desirable to provide a flow impedance device for a flowmeter which overcomes the disadvantages associated with the standard single orifice design. More specifically, it is desirable to eliminate the requirement of flow conditioning and to reduce or minimize any pressure drop across the flow impedance device without sacrificing the accuracy of the resulting flow rate measurement.